Radiation treatment has been widely used for clinical treatment for several decades. An imaging process is often required for the radiation treatment. For example, image-guided radiation therapy (IGRT) involves obtaining an image by a scanning operation to direct radiation treatment. The imaging process may also generate images as a record of treatment. Radiation output for imaging is usually much lower than radiation output for a treatment. The same dosimeter may be used to measure the radiation output for the imaging and the radiation output for the treatment. Such a dosimeter may be designed to pass as much radiation as possible and may thus exhibit low quantum efficiency. As a result, the dosimeter may have a limited ability to measure radiation output for imaging operations. Knowledge of the radiation output per imaging frame is important both for recording the imaging dose and for the normalization of an image intensity to the radiation output for the imaging. This is particularly important for computed tomography, in which severe image artifacts may result the projection images are produced by a variable and unknown amount of radiation per image. The conventional radiation detector used for measuring radiation output by the therapy/imaging system has, by itself, limited ability to detect changes in beam energy. A secondary radiation detector may be used to improve safety, by detecting changes in beam energy (quality) and provide a means of calibrating radiation beam energy spectral characteristics. Therefore, it may be desirable to develop devices and methods for measuring the radiation output rate and monitoring beam energy more accurately and more precisely for radiation therapy or an imaging process.